Ubisoft has released a hotfix to help stop players using an exploit "to force themselves into squads with other players" in Rainbow Six Siege.
The change is in response to a recent spike in hackers forcing their way into private lobbies and refusing to leave, and there's currently no way to kick players from your squad. Right now, it's only been deployed on PC.
"To fix an exploit where players could join lobbies uninvited, we have released a hotfix that will require PC players to have at least one friend already in the squad," explained a tweet on the official Rainbow Six Siege Twitter account. "Thank you for all your patience!
"To clarify, now players can only join games in which they have a friend already in that squad (whether it be the squad host, or someone else already in that party)," the account clarified a short while later. "Players can no longer use an exploit to force themselves into squads with other players."
Source